Field of the Invention
The present invention relates to a method for manufacturing a honeycomb structure capable of using as an exhaust gas purification filter and relates to a honeycomb structure.
Description of Related Art
Heretofore, honeycomb structures have been used as a trapping filter to remove particulate matter (PM) discharged from a diesel engine or the like. As a honeycomb structure used as a trapping filter for particulate matter, there has been used a plugged honeycomb structure provided with plugged portions at predetermined position of the both end faces.
Herein, a plugged honeycomb structure includes a honeycomb structure part having porous partition walls defining a plurality of cells serving as through channels of fluid, and plugged portions provided with an end part of a predetermined cell (first cell) on the fluid inflow-side and with an end part of a remaining cell (second cell) on the fluid outflow-side. These plugged portions are generally arranged so that the first cell and the second cell are alternately arranged to form a so-called checker board pattern. According to such a plugged honeycomb structure, when exhaust gas flows into the cells from the end face on the exhaust gas inflow side, the exhaust gas flowing into the cells passes through the partition wall. Then, as the exhaust gas passes through the partition wall, the PM contained in the exhaust gas is trapped by the partition wall. Therefore, the exhaust gas passing through the partition wall is discharged as purified gas.
The plugged honeycomb structure is prepared by filling a plugging material serving as a material of the plugged portions into cell openings of the honeycomb formed body, which is formed into a honeycomb shape with a use of kneaded material, followed by firing. A ceramic raw material is contained both in the kneaded material serving as a material of the honeycomb formed body and in the plugging material serving as a material of the plugged portions, but there is a difference in firing shrinkage rate between the honeycomb formed body and the plugging material during firing.
As a case where there is a difference in firing shrinkage rate between the honeycomb formed body and the plugging material during firing, it can be divided broadly into a case, where the shrinkage of the honeycomb formed body is larger than that of the plugging material during firing, and the opposed case, where the shrinkage of the plugging material is larger than that of the honeycomb formed body during firing.
When the honeycomb formed body shrinks more than the plugging material, in the honeycomb structure obtained after firing, the shrinkage at end face part having a plugged portion is smaller than the central part having no plugged portion. This is because that a plugging material does not shrink so much as the honeycomb formed body during firing, and hence the end face part of the honeycomb formed body is constrained by the low shrinkage of the plugging material. Therefore, a diameter at the end face part of the honeycomb structure becomes larger than that at the central part. As a result, honeycomb structure should have originally in cylindrical shape, but it becomes a concave drum shape. When the deformation of such a concave drum shape is remarkable, it becomes impossible to keep the diameter of the honeycomb structure within predetermined dimensional tolerance. Therefore, it becomes necessary to grind the outer periphery of the honeycomb structure to a cylindrical shape after firing, and then to perform the outer coating.
In addition, when the honeycomb formed body shrinks more than the plugging material, the end face part of the honeycomb formed body is constrained by the low shrinkage of plugging material as stated above, and hence compressive stress remains in the end face part of the honeycomb structure obtained after firing. However, this residual compressive stress hardly causes a problem of decrease in thermal shock resistance in the honeycomb structure. This is because that, when the honeycomb structure is used as a filter for trapping particulate matter, the compressive stress remaining in the end face part is offset by tensile stress generated at the time of burning and removing the deposited particulate matter.
On the other hand, when the plugging material shrinks more than the honeycomb formed body, a plugging material excessively shrinks at the end face part. Therefore, a gap is often generated between the plugged portion and the partition wall at the end face part of the honeycomb structure obtained after firing. Such a gap may cause the lacking of the plugged portion or cause the leakage of particulate matter when the honeycomb structure is used as a filter for trapping particulate matter. Moreover, when the plugging material shrinks more than the honeycomb formed body, there is a case where the aforementioned gap is not generated, but in such a case, the end face part having the plugged portion shrinks more than the central part having no plugged portion. This is because that plugging material shrinks more than the honeycomb formed body during firing, and hence the end face part of the honeycomb structure is constrained by the high shrinkage of the plugging material. As a result, the honeycomb structure should have originally in cylindrical shape, but it becomes a barrel shape. When the deformation of such a barrel shape is remarkable, it becomes impossible to keep the diameter of the honeycomb structure within predetermined dimensional tolerance. Therefore, it becomes necessary to grind the outer periphery of the honeycomb structure to a cylindrical shape after firing, and then to perform the outer coating.
In addition, when the plugging material shrinks more than the honeycomb formed body, the end face part of the honeycomb formed body is constrained by the high shrinkage of plugging material as stated above, and hence tensile stress remains in the end face part of the honeycomb structure obtained after firing. When a honeycomb structure is used as a filter to trap particulate matter, high heat is generated inside the honeycomb structure at the time of burning and removing the deposited particulate matter. The honeycomb structure is expanded due to this high heat, and hence high tensile stress is generated in the honeycomb structure. As stated above, when tensile stress remains in the end face part of the honeycomb structure by a difference in firing shrinkage rate between the honeycomb formed body and the plugging material, it would be overlapped to the tensile stress resulting from the burning of the particulate matter. Thus generated overlapped tensile stress might be exceeded the structural strength of the honeycomb structure. Consequently, when the plugging material shrinks more than the honeycomb formed body, thermal shock resistance might be reduced in the honeycomb structure obtained after firing.
In order not to reduce the thermal shock resistance of the honeycomb structure, it is needed to limit the difference in firing shrinkage rate between the plugging material and the honeycomb formed body to a predetermined value or less. Under such a background, there has been proposed a method for manufacturing a plugged honeycomb structure, wherein a difference in firing shrinkage rate between the honeycomb formed body and the plugging material is 7% or less (Patent Document 1).    [Patent Document 1] WO 2004/085029